#include<bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  long long fact = 1;
  for(int i=1;i<=n;++i) {
    fact *= i;
    if(fact>1000000000) fact %= 100000000;
  }
  int cnt=0;
  while(fact%10==0) {
    fact/=10;
    cnt++;
  }
  // cout << fact << endl;
  cout << cnt << endl;
  return 0;
}
